This is the same machine involved in this question: ender-3-pro-extruder-stepper-skipping-with-a-chunk-sound.
After reassembling the extruder, I discovered that the extruder stepper did not move, using prepare->move-axis. All three axis steppers did move normally.
The first thing is did to test was to exchange the wires to the extruder and X-axis steppers. After this the extruder stepper moved when I moved the X-axis but the X-axis did not move when I moved the extruder. Therefore, I concluded that the stepper is working.
I then opened up the main board chassis and exchanged the X-axis and extruder cables there (so the cables are exchanged on both ends). This restored operation to the X-axes (operated as such) and the extruder did not function again (operated as such).
From this, I concluded that the cables are also fine and I had somehow blown the extruder driver on the board.
I went on Amazon and ordered a "silent" Ender 3 Pro motherboard, rev 1.1.5 to replace my current rev 1.1.4 board. I just received the new board and, as far as I can tell, it looks like an authentic Creality product. The only visible differences between the old and new boards are the silkscreened version and the color of the PCB itself. Both are labelled "Ender 3 Pro" on the back.
I carefully removed the old board, marking where all cables connect and swapped in the new board. Much to my distress, everything acted exactly the same. The extruder stepper did not turn. Swapping the cables at the steppers restored functionality to the extruder (controlled as X-axis) and the X-axis did not move (controlled as extruder). Also swapping the cables at the motherboard restored the X-axis stepper (controlled as such) but the extruder stepper would not move (controlled as extruder).
I should add for Completeness that my printer was sold by Sain Smart (Creality OEM, I believe) and was labelled as such, both on the metal and on the LCD start screen. However, the motherboard was a Creality3D board, labelled as "Ender 3 Pro" and I believe the only actual difference is the text inside the firmware. With the new board in, the LCD identifies itself as "Creality Ender 3 Pro".
At this point, the only conclusion I can come to is that the new board has failed in the same way, which doesn't make any sense.
I need a sanity check. Does anyone see any flaw in my logic?
I should add that there is an oddity on the new board. The fan on front of the hot end seem to run at full speed and the hot end side fan and chassis fan run briefly at power up an then stop. Using control->temperature->fan-speed seems to have no effect on any of the fans. I never observed the chassis fan with the old board so I cannot say this is different but I did previously have control of the hot end fan.